Oil country tubular goods, line pipes, heat exchanger tubes, general pipes, and pipes for bearing rings, are usually made of seamless steel pipe. Seamless steel pipe used for such purposes is typically made of carbon steel, low alloy steels containing alloy components such as Cr and Mo, and high Cr stainless steels. Seamless steel pipe is usually manufactured by the Mannesman-mandrel mill method. However, this method is often very complicated because, for example, severe hot working is performed by a piercer and a high level of properties is required of the resulting products.
FIG. 1 shows an example of a manufacturing process employing a Mannesman-mandrel mill method. There are a number of steps in forming a starting steel ingot into a pipe product. The material being processed undergoes various types of working, heating, and cooling in a repeated manner. The broken line in FIG. 1 indicates changes of lines which entails transfer of materials between steps and processing such as temporary stocking. In a manufacturing process employing a Mannesman-mandrel mill method, a lot of process lines are employed, requiring many types of equipment having advanced functions and consuming a large amount of energy. This results in an unavoidable increase in costs.
In order to reduce manufacturing costs, it is necessary to enhance productivity, to cut equipment costs, and to reduce operating costs. More specifically, in the manufacture of seamless steel pipe, it is desired to simplify manufacturing steps and equipment and to obtain products having properties superior to conventional ones.
To this end, a variety of techniques have been developed for the manufacture of seamless steel pipe. Particularly, many proposals have been made concerning forming a billet from a steel ingot, hot piercing, elongating, finish rolling, and subsequent heat treatment for providing predetermined properties to the product after finish rolling.
With respect to the step of making a billet having a round cross section from a steel ingot, there is proposed an approach in which a round billet is produced by continuous casting so as to avoid blooming or forging. For example, Japanese Patent Application Laid-open (kokai) No. 63-157705 discloses a method for manufacturing a seamless steel pipe in which a billet having a round cross section is pierced and then elongated. In the method disclosed in this publication, however, technical problems with respect to heating conditions for billet piercing and piercing conditions of a piercer (i.e., a skew-roll piercing mill) are not sufficiently solved. Therefore, a material pierced according to this method tends to generate cracks during piercing.
Also, from the viewpoint of performing process steps in a continuous manner, "Iron and Steel" vol. 71 (1985) No. 8, pp.965-971 discloses a manufacturing facility in which a mandrel mill (which is a continuous elongating mill) and a sizer (which is a finish rolling mill) are directly connected. The reason for directly connecting a continuous elongating mill and a finish rolling mill as given in this publication is only to secure a quenching temperature. As a natural consequence, the material which has undergone finish rolling is subjected to quenching while it still is at a high temperature, resulting in coarsening of grains and a reduction in toughness of the resulting pipe product.
Many improvements have been proposed with respect to the heat treatment step for imparting to the material predetermined properties which must be possessed by final products. Seamless steel pipe is required to have highly reliable quality and excellent properties. Therefore, as shown in FIG. 1, heat treatment, including quenching and tempering which are critical to the product quality, is normally performed off-line because this enables strict line control. That is, a quenching apparatus and a tempering furnace are normally provided independently of the pipe-forming line. Such off-line processing hampers simplification of manufacturing facilities and reductions in energy consumption.
To solve this problem, there has been an attempt in recent manufacture of seamless steel pipe to perform on-line quenching by a so-called direct quenching method exploiting heat of a pipe which has undergone finish rolling. The direct quenching method is advantageous in that it renders off-line quenching equipment unnecessary and simplifies manufacturing steps, achieving a considerable reduction in costs.
For example, Japanese Patent Application Laid-open (kokai) Nos. 56-166324, 58-120720, 58-224116, 56-020423, 60-033312, 60-075523, and 62-151523 disclose a method for manufacturing seamless steel pipe including direct quenching in which steel pipe is forcibly cooled immediately after it passes a finish rolling step. Unfortunately, products obtained through a process including direct quenching do not have as good quality as those obtained through a process in which off-line quenching is performed. In other words, grain in microstructure is coarser than those obtained through conventional processes, and therefore, toughness and corrosion resistance are inferior.
In order to refine grain, an on-line thermomechanical treatment has been proposed. For example, Japanese Patent Application Laid-open (kokai) No. 56-003626 discloses a method in which a cooling step and a reheating step are incorporated between elongating and finish rolling. Japanese Patent Application Laid-open (kokai) Nos. 58-091123, 58-104120, 63-011621, and 04-358023 disclose a method in which a treatment combining cooling and reheating is performed after finish rolling. Japanese Patent Application Laid-open (kokai) No. 58-117832 discloses a method in which cooling and reheating are performed twice, the first time being in the course of rolling (between elongation and finish rolling), and the second time being after finish rolling. Any of these methods employs an on-line combination of cooling and reheating and features a total of two or more iterations of transformation from austenite to ferrite and transformation from ferrite to austenite.
Any of the above methods requires that a pipe material to be processed be forcibly cooled to a temperature range in which transformation commences or is completed and be subsequently reheated to a temperature range in which austenitization is completed. Thus, these methods consume large amounts of energy, causing high energy costs. In addition, they require complicated manufacturing equipment, increasing construction costs for manufacturing facilities. Moreover, mechanical properties such as strength, etc. of seamless steel pipe manufactured by a direct quenching method greatly inconsistent. This is because the quenching temperature is not uniform in the longitudinal direction of a steel pipe or because the temperature differs between manufacture lots. Therefore, there are still problems to be solved in order to efficiently mass produce seamless steel pipe having a uniform quality. Thus, the above-mentioned methods require improvements in equipment costs and operating costs and also in properties of resultant products when compared with conventional methods involving off-line quenching.
In the manufacture of seamless steel pipe, if respective steps are arranged off-line in an independent manner, there arises a problem ;hat space is needed for storing billets and like materials to be processed because processing speed differs from step to step. For example, since a billet yard for storing billets to be pierced and a place for temporarily storing steel pipe before heat treatment are needed, a large area is required. Also, in order to convey materials from step to step, a number of conveyances including auxiliary means such as cranes, trucks, etc. are needed.
As described above, none of the above-described conventional methods were successful in manufacturing seamless steel pipe having excellent properties using simplified manufacturing steps and equipment with high productivity and reduced manufacturing costs.
The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing seamless steel pipe having properties superior to those of conventional products using simple manufacturing steps and equipment at reduced costs with good productivity, and a manufacturing facility for performing the method.